8-Speed Transmission Noise Concerns

There may be a noise concern on some 2015-2019 Corvette; 2015-2020 Escalade, Yukon; 2015-2024 Silverado 1500, Sierra 1500; 2016-2018 CT6; 2016-2019 ATS, CTS; 2016-2023 Camaro; 2017-2023 Colorado, Canyon; 2017-2024 Express, Savana; and 2020-2024 CT4 models equipped with an 8-speed automatic transmission (RPO M5N, MNU, M5X, MHA, M5T, M5U, MQE, MQD). (Fig. 15) The sound may be heard coming from the rear of the engine or the front of the transmission.

 

Fig. 15

 

Begin diagnosis by verifying the origin of the noise under the vehicle. Raise the vehicle and, with the engine running, use a stethoscope to determine where the noise is coming from — either the rear of the engine or the front of the transmission. If needed, a hose can be attached to the vent of the transmission and brought inside the cabin of the vehicle to pinpoint if the noise is coming from the transmission.

TIP: Do not unbolt the torque converter from the flexplate as this could lead to damage to the engine and transmission as well as result in an incorrect diagnosis.

Conditions to Check Before Disassembly

Once the noise is confirmed to be from the front of the transmission, verify the following items prior to removing the unit for inspection:

• If equipped with the 2.7L turbo engine (RPO L3B), make sure to follow Bulletin #24-NA-078. If the noise is found to be coming from the starter assembly, replace the starter and re-evaluate the concern. The starter noise may be intermittent, and the noise can begin after the engine is running or following a remote start.
• If the vehicle is equipped with the 4.3L engine (RPO LV3), 5.3L engine (RPO L82, L84), 6.2L engine (RPO LT1, LT4, L87) or 6.6L engine (RPO L8T), follow Bulletin #23-NA-170 and verify the noise is not coming from the engine or a damaged thrust bearing. There have been instances of transmissions replaced for engine noise, and often this noise can appear to be coming from the front and/or middle of the transmission based on the transfer path.

If there are not any conditions found with the starter or the engine, the transmission will need to be removed and inspected.

Verify Endplay Measurement

Make sure to verify the transmission endplay measurement using the required special tools and following the proper procedure in the appropriate Service Information prior to replacing the torque converter and prior to disassembly of the unit. (Fig. 16) The endplay measurement is critical to proper diagnosis. There have been instances of a worn, damaged or missing transmission thrust bearing and/or other components that may be easily overlooked.

 

Fig. 16

 

If a noise concern is found internal to the transmission, disassembly of the transmission and a detailed inspection will be required to verify there are not any missing or damaged components. Some internal components can be damaged or missing and not affect the endplay measurement.

For additional information, refer to #PIP5996A.

– Thanks to Bill Alley